The oscillation which is made by a point of the earth, because of an earthquake, occurs in three dimensions and is different for each earthquake. In other words, this oscillation is expressed in a three--dimensional coordinate system OXYZ comprised of three components, i.e., the oscillation consists of components along the three axes OX, OY and OZ. With the axis OZ positioned vertically, the component of the oscillation of the earthquake along this axis is called the vertical component and the components along the OX and OY axes are called horizontal components. Seismometers, the instruments which record earthquake movements, record all three of these components. However, the relative importance of the respective components is not equal in relationship to the safety of structures. It has been generally accepted that the vertical component usually is less important than the horizontal components, because the capacity of the structure to transfer vertical loads is generally far greater than for horizontal loads. This is because they are shaped in a way suitable for transferring of the vertical loads of the structure and because the capacity of the ground is generally far greater for vertical forces than for horizontal forces. In addition, it has been presumed that the possibility of a vertical component OZ of the seismic accelerations greater than 1.0 g is very small because it has been presupposed that the soil and the surface which supports the structure is generally not capable of transferring tension forces.
Because of these factors, the science of earthquake engineering has directed its attention primarily to the horizontal components and has given only secondary consideration to vertical components. However, in the earthquake which occurred in Imperial Valley, Calf., United States, in 1979 one accelerometer (Station 6) recorded a vertical component of 1.74 g. The reason for this occurrence is not yet adequately understood. However, that particular station was located within a few kilometers of the fault, and in fact was between two faults which were disturbed during the eqrthquake. It is possible that the earth was horizontally compressed at that location by simultaneous disturbances along both faults, causing a great vertical acceleration. Nevertheless, this occurrence makes it clear that vertical components of earthquakes will require further consideration, and that isolation of structures from all the components, both horizontal and vertical, will be desirable at least in some cases. The cases in which such isolation may be necessary will be known better when it becomes more clear what conditions cause vertical accelerations of significant magnitude.
In my U.S. Pat. No. 4,166,344, I described a system for isolating a structure from the earth when horizontal seismic accelerations exceeded a predetermined value. The system includes a disc or load distributing base, usually of reinforced concrete, supports which carry the weight or vertical load of the structure and which also impose elastic restoring forces on the structure which react against horizontal movement, and connecting means which prevent horizontal movement of the construction under normal circumstances, for example, when the building is subjected to forces from winds. The connecting means are designed to disconnect when the structure is subjected to horizontal forces greater than a predetermined value.
Until new structural materials are invented, with properties which are adequately different from the properties of existing materials, something which cannot be foreseen, it is also absolutely necessary to avoid certain structural elements in order to avoid instability of the structure. The elements to be avoided are those which transfer great axial loads in a first direction and which are simultaneously required to follow great deformations in a second direction which is perpendicular to the first one. For this reason, it is necessary to provide separate elements for the vertical and horizontal isolation.